1. Field
This disclosure relates generally to semiconductor devices, and more specifically, to semiconductor devices having source/drain stressors.
2. Related Art
Source/drain stressors have been developed to provide strain in channel regions to improve transistor performance. Tensile stress applied to the channel has been found to improve electron mobility for N channel transistors while compressive stress applied to the channel has been found to improve hole mobility. The degree of improvement is generally greater with greater stress being applied. The source/drain stressor approach involves removing the semiconductor material near the channel area to form recess regions there and then filling recess regions by growing a semiconductor material of a different type. With silicon being the starting semiconductor material, which is typical, the tensile stress can be exerted by growing silicon carbon and the compressive can be exerted by growing silicon germanium. One limitation on the stress is the carbon and germanium concentrations. Increasing these concentrations increases the stress but also increases the likelihood of dislocations. Dislocations reduce the stress. So the carbon and germanium concentrations are as a large as possible that do not result in forming dislocations. Transistor performance, however, would be improved with further increases in strain without creating other problems such as increasing transistor leakage.
Thus, there is a need for further improving the performance of devices with source/drain stressors.